What about the distributor cap? Not sure about the 350, but that's the first thing I check when my 427's start acting wonky and that fixes the problem 90% of the time.
10 to 30% chance, how old are the coils?
float carbs level ! and look your fuel pump psi !! i put a fuel pressure gauge on my fuel line and i keep it to 7 pounds ,to much pressure its not good too... ..